Hi everyone! I've been playing with some fans I had lying around and was interested in learning how to use the tach of the fans. I've read many articles to use either Pin2 and/or Pin3 of the arduino and use interrupts. That would only allow me to read 2 fans max. I was wondering if I could use the analog inputs (with a low-pass filter) to read the fan tachs. I do not care too much about the accuracy. I'm more interested in just being able to know if the fan is still working (or spinning). What would you guys suggests for something like this?? Thanks in advance!

Approx. what is the fan tach frequency? It's often a fraction of the fan RPM. You may be able to use the millis() or micros() to measure the delay between tach pulses (period) and then convert the period to frequency (f = 1/t) . An RC network (integrator) on each fan tach and analogRead() to measure the output voltage from the network would work, which essentially converts the pulse rate to a voltage and won't be terribly accurate but should be good enough for what you want.

If you're not terribly concerned about absolute accuracy, pin change interrupts should work. Especially if you just want to know that they are running. Many pins can support the PCINT. Inside the ISR you'd just need to determine the pin(s) that caused an interrupt and set flags that main level checks. If main level processing sees a flag going unset for x amount of time (or y loop iterations), then the fan must not be running.